1. Field of the Invention
The present invention relates to a knee-action-type suspension applied to a vehicle such as a car, and a compression coil spring for suspension (hereinafter referred to as a suspension coil spring).
2. Description of the Related Art
Patent Document 1 (Jpn. Pat. Appin. KOKAI Publication No. 2004-50906) discloses a knee-action-type suspension which is used in a suspension mechanism part of a vehicle such as a car. This type of suspension comprises an arm member, a compression coil spring which serves as a suspension spring, a lower spring seat, an upper spring seat, a shock absorber which restrains a vertical shock of the arm member, and the like. The arm member is supported in such a way that it is pivotable in the upward and downward directions by a pivot provided in a vehicle body. The lower spring seat is disposed on the lower side of the compression coil spring. The upper spring seat is disposed on the upper side of the compression coil spring.
In this field, from a standpoint of reducing the weight of a vehicle, reducing the weight of a suspension compression coil spring is strongly desired. It is known that in a suspension compression coil spring, a stress which is produced in each part of a wire is generally not constant in a state where a load is applied. In order to reduce the weight of such a compression coil spring, it is effective to approximate a stress distribution of the wire to a uniform distribution (i.e., make it flat) as much as possible. As one way of uniforming the stress distribution of a compression coil spring, varying the wire diameter in one turn of a wire has been proposed, as described in Patent Document 2 (Jpn. Pat. Appin. KOKAI Publication No. 59-219534). For example, in a coil spring in which an external force acting point is offset in a radial direction of the coil, reducing the diameter of a wire on the offset side has been proposed. Alternatively, in a coil spring to which a load is applied obliquely with respect to the coil central axis, forming a portion having a large wire diameter and a portion having a small wire diameter alternately in the direction of the coil central axis has been proposed.
The inventors of the present invention have studied ways to uniform a stress of a compression coil spring used in the knee-action-type suspension. For example, in a suspension in which the arm member is pivoted up and down about a pivot, it has been suggested from the past to reduce the diameter of a wire which is on the side close to the pivot, or form a portion having a large wire diameter and a portion having a small wire diameter alternately in the direction of the coil central axis. However, with such conventional technology, depending on the position of the arm member when it is moved up and down, it has been found that variations in the stress distribution of the wire are more significant on the contrary, instead of making the stress distribution more uniform.